MART� MUS, M.*; BERGSTR�M, J.: The skeleto-muscular system of hyolithids

Hyolithids are Palaeozoic bilaterians with a calcareous exoskeleton consisting of a conical conch, an operculum and a pair of long, curved “spines” called helens. They belong to the group Hyolitha, which includes other operculate, conical forms lacking helens. Hyoliths were protostomes likely related both with molluscs and sipunculans. A close relationship between these latter groups has been proposed on the basis of biological data. The phylogenetic position of hyoliths may lie in the stem lineage of either of these groups or in the lineage leading to both. The morphology of hyoliths may therefore shed light on the organisation of the common ancestor of molluscs and sipunculans. The skeletal elements of hyolithids were connected by soft tissues and are easily disarticulated after death; this and the fragility of the helens have hindered knowledge of the morphology of the complete organism. A variety of muscle scars occur on the conch and operculum and have played a central role in controversial reconstructions of the internal organisation of hyolithids. The study of exceptionally preserved hyolithids has provided new insights into the organisation of the hyolithid skeleton. The helens were solid and grew by terminal accretion. In life position, they curved ventrally and posteriorly. Upon closure of the operculum, each helen was locked between the margins of conch and operculum with its dorsal edge tilted forwards. Helens were involved both in locomotion and stabilisation and the need to fulfil these two functional requirements may explain their inefficient appearance. Comparative study of muscle scars in different species indicates that hyolithids shared a common muscular system consisting of discrete, non-seriated muscle bundles operating the articulation of the skeletal elements.